Optical near-field microscopy is based upon the measurement of scattered light at a near-field probe which is generated by optical near-field interaction between the near-field probe and a sample. To achieve high local resolution known (near-field) probes comprising sharp tips are used, e.g. such probes as used in atomic force microscopy. The (near-field) probe is illuminated at its tip by focused light, e.g. in the visible or mid-infrared spectrum, to generate scattered light during tip-specimen interaction. The optical near-field of the sample is typically determined by scanning (scan-probing) the sample with the probe. From the measurement of the light scattered by the probe, in particular the near-field signal, material properties of the sample can be obtained with a local resolution down to nanometer scale without limitations imposed by diffraction of light.
An apertureless near-field optical microscope is disclosed in EP 394 668 B1.
The light scattered by the tip of the probe (in the following only “probe”) is collected since it conveys the information on the local optical properties of the sample. The presence of a sample (also referred to as specimen) in close proximity to the tip modifies the scattered light amplitude and phase because the scattering depends not only on the tip alone, but on the polarizability of the entire coupled probe-sample system. The optical resolution of the near-field microscope is essentially limited only by the tip radius.
The general problem of scattering type near-field optical microscopes is that the largest part of the collected light does not originate from the tip apex. Instead, it is mostly produced by reflections and scatterings from the tip shaft and the entire illuminated area of the sample. This undesirable part of the signal is commonly referred to in the art as background signal, or background light. Several methods to avoid the background signal are known in the art.
EP 1 770 714 A1 discloses a method for reducing the background signal by demodulating the scattered light at the frequency of the higher harmonics of the tip oscillation. This way, the near-field signal to background signal ratio can be significantly improved, as indicated in FIG. 1. While the unmodulated background signal (B0) is significantly larger than the unmodulated near-field signal (N0), the near-field signal at the first demodulation order (N1) and the background signal at the first demodulation order (B1) are approximately of the same order of magnitude. At the second demodulation order the near-field signal (N2) becomes significantly larger than the background signal (B2). However, as indicated in FIG. 1, by using the second demodulation in order to suppress background signal, the useful signals N0 and N1 are lost, which is of disadvantage as N0 is typically 10 to 100 times higher than N2, and N1 is typically 3 to 10 times higher than N2. Higher demodulation orders lead to even higher loss of near-field signal.
A further method for reducing background interference is disclosed in DE 10 035 134. The disclosed method is based on the detection of the scattering at higher harmonics of the tip oscillation frequency, heterodyned with the reference wave shifted by a specific frequency in respect to the light used for tip and sample illumination. This heterodyne method has the disadvantage that the frequency shift required for heterodyning is produced by an acousto-optical modulator (AOM) which separates the shifted beam only by a small angle from the unshifted beam at its output. The small shift of the modulated beam provides difficulties in the alignment of the light paths. Furthermore, AOMs are expensive and commercially available only for a few wavelength ranges which strictly limits their near-field microscopic, and especially near-field spectroscopic applications.
Therefore, there is still a need in the art for an improved method for measuring the near-field signal of a sample in a scattering type near field microscope which does not show the above mentioned problems of the prior art.